This invention relates generally to an electrostatographic printing machine, and more particularly concerns an improved development system for use therein.
An electrostatographic printing process forms an electrostatic latent image of an original document and reproduces the image in viewable form on a copy sheet. Electrostatography includes electrophotography and electrography. As is well known, electrophotography employs a photosensitive medium to form, with the aid of electromagnetic radiation, the electrostatic latent image. Contrawise, electrography utilizes an insulating medium to form, without the aid of electromagnetic radiation, the electrostatic latent image. Both of the foregoing processes render the latent image viewable by the process of development, i.e. depositing particles thereon. Frequently, the particles are transferred from the latent image to a copy sheet. Alternatively, the recording sheet on which the latent image is produced may also serve as the copy sheet after the particles have been deposited thereon. In both of the foregoing cases, the resultant toner powder image deposited on the copy sheet is permanently affixed thereto by the application of heat and/or pressure. Hereinafter, an electrophotographic printing machine will be described as an illustrative embodiment.
Electrophotographic printing employs a photoconductive manner which is charged to sensitize the surface thereof. The charged portion of the photoconductive member is exposed to a light image of the original document being reproduced. Exposure of the sensitized portion of the photoconductive member discharges the charge therein selectively in the irradiated areas in accordance with the intensity of the light transmitted thereto. This creates an electrostatic latent image on the photoconductive member corresponding to the original document being reproduced. Development of the electrostatic latent image recorded on the photoconductive member is achieved by bringing a developer material into contact therewith. Generally, the developer material comprises dyed or colored heat settable plastic powders, known to the art as toner particles, which adhere triboelectrically to coarser carrier granules, such as ferromagnetic granules. The toner particles and carrier granules are selected such that the toner particles have the appropriate charge relative to the electrostatic latent image recorded on the photoconductive member. Thus, when the developer material is brought into contact with the latent image recorded on the photoconductive member, the greater attractive force thereof causes the toner particles to transfer from the carrier granules and adhere to the electrostatic latent image. This process was disclosed originally by Carlson in U.S. Pat. No. 2,297,691 and is further amplified and described by many related patents in the art.
The process of electrophotographic printing was significantly advanced with the advent of multi-color electrophotographic printing. In color electrophotographic printing, filters form single color light images from the colored original document. Each single color light image records a single color electrostatic latent image on the photoconductive member. The single color electrostatic latent image is developed with toner particles of a color complementary to the color of the filtered light image. Thereafter, each differently colored toner powder image is transferred to the copy sheet in superimposed registration with one another. This results in a multi-layered toner powder which is subsequently permanently affixed to the copy sheet. In this manner, the copy sheet has a transparent multi-layered toner powder image formed thereon. Each layer is of a different color. Light rays pass through the multi-layered powder image and reflected from the copy sheet back through the layers to the eye of the observer. Each layer acts as an optical colored filter so that the observer sees an image having a composite color. This results in the observer seeing a copy corresponding in color substantially to the original document being reproduced. U.S. Pat. No. 3,854,449 issued to Davidson in 1974 describes this process in greater detail.
Recently, it has been highly desirable to create flat color, i.e. a copy containing colored information formed as a single layer rather than a multiple layer. This may be achieved by utilized colored toner particles corresponding to all desired colors. This requires that the developer housing storing the toner particles be replaced or differently colored toner particles placed therein to change copy colors. Alternatively, subtractive primary color toner particles may be blended in the developer housing. The ratios of toner particles defines the resultant color of the mixture. In order to vary the color of the mixture, the ratios of the substractive primary color toner particles must be suitably adjusted. Thus, the development system comprises a mixture of cyan, magenta, and yellow toner particles. The ratios of respective toner particles produces a resultant mixture of toner particles having the desired color.
A development control system which regulates the concentration of toner particles in each developer housing of a multi-color electrographic printing machine is described in U.S. Pat. No. 3,754,821 issued to Whited in 1973. This patent describes a control system for regulating the concentration of toner particles in each developer housing employed in a multi-color electrophotographic printing machine. Each developer housing is controlled independently and contains only single color toner particles therein.
Another multi-color electrophotographic printing machine is described in U.S. Pat. No. 3,910,232 issued to Kondo et al in 1975. This patent teaches that developing agents employed in a multi-color development system may be re-used as a black color developing agent. This is achieved by mixing the developing agents in a recovery unit. These differently colored developing agents are mixed with a black colored developing agent so as to be suitable for monochromatic reproduction. The combined developing agent may be utilized for a monochromatic reproduction that is not necessarily black.
Accordingly, it is a primary object of the present invention to improve the development apparatus employed in an electrophotographic printing machine so as to be able to form a mixture of differently colored particles to achieve a resultant pre-selected color therein.